Under circumstances in which a vehicle AC (air conditioning) system runs low on refrigerant (known as a low refrigerant “charge”), lubrication of the AC compressor may be reduced, potentially causing damage to the compressor. Additionally, the refrigerant discharge temperature may rise, also potentially damaging the compressor. These factors may necessitate expensive repair or replacement of the compressor. The repair or replacement expense may be particularly high in the case of an electric compressor.
Current methods of detecting a low A/C system refrigerant charge may include utilization of a low side pressure switch and a continuous saturation pressure check. However, these methods may not be highly effective in detecting a low state of refrigerant charge under a number of conditions. For example, a low side pressure switch can be set to trip when the suction pressure reaches a lower limit and sends a signal that shuts down the compressor until the pressure climbs above an upper pressure limit. The saturation pressure check occurs on a continuous basis and uses an ambient temperature sensor to determine the saturation pressure of R-134a at that temperature and compares it to the current system head pressure, measured with a transducer. If the head pressure is below the saturation pressure at a given temperature, then the compressor is disabled.